An example of a radio communication system will be explained in which a plurality of adjacent radio zones are formed by locating a plurality of base stations in respective radio zones.
FIG. 1 depicts a diagram illustrating an exemplary cell allocation of three base stations BS1 to BS3. In FIG. 1, a state of the zonal frequency allocation is illustrated when FFR (Fractional Frequency Reuse) in the frequency axis direction is applied to improve frequency use efficiency in particular.
FIG. 2 illustrates exemplary frequency allocation when FFR in the frequency axis direction is applied. In FIG. 2, for radio communication with terminals or mobile stations located in the vicinity of the center in a radio zone of each base station BS1-BS3, an identical frequency band A [such as depicted in (B) of FIG. 2] is used. Also, in the peripheral areas in the radio zone of the respective base stations, frequency bands B, C and D obtained by dividing the above frequency band A into three [such as depicted in (A) of FIG. 2] are used. As an example in FIG. 1, the base station BS1 uses the frequency band B.
Using such the frequency allocation, it is possible to mix the merit of frequency allocation producing optimal frequency use efficiency (which is referred to as reuse 1) with the merit of frequency allocation producing improved throughput by reducing interference (which is referred to as reuse 3).
Namely, since an identical frequency may be used in the vicinity of the center in the radio zone of the base station because of low interference, the frequency allocation by the reuse 1 is made. On the other hand, in the peripheral area in the radio zone of the base station, since the reuse 1 using an identical frequency produces large interference, causing reduction of throughput, different frequencies are used by the reuse 3. By this, it is possible to improve the throughput by applying the reuse 1 producing the highest frequency use efficiency, while interference may be reduced in the peripheral area.
In FIG. 3, an exemplary frame structure when FFR in the frequency axis direction is applied in the typical OFDM (Orthogonal Frequency Division Multiple Access) is illustrated. In the figure, the vertical axis indicates a subchannel direction and the horizontal axis indicates a symbol direction. The example in FIG. 3 depicts frame mapping in regard to the base station BS1. In the reuse 3 zone of the symbol direction (time axis direction), a ⅓-divided frequency band is allocated to each of the three base stations.
On the other hand, FIG. 4 illustrates a frequency allocation diagram in case that FFR is applied in the time axis direction. The zonal frequency allocation is the same as the allocation illustrated in FIG. 1. However, a common frequency band is applied to each base station BS1-BS3, as illustrated in FIG. 4.
FIG. 5 illustrates an exemplary OFDMA frame structure in the case that FFR in the time axis direction is applied in correspondence to FIG. 4. The period of an applied frequency in a frame is allocated by being time divided into ⅓, which is a different point from FFR in the frequency axis direction illustrated in the above FIG. 3.
As a related art, there is an invention disclosed in patent document 1, Japanese Laid-open Patent Publication No. 2007-274042. According to the above patent document 1, the transmission power of a base station is varied or the magnitude of reuse 1 and reuse 3 zones is varied on the basis of a data amount.